In a known Long Term Evolution (LTE) system, a concept known as Admission Control has been implemented in order to admit or reject a request for resources for establishing connections for carrying data traffic. When the request is admitted, the connection is established in the form of a radio access bearer such as an Evolved-Radio Access Bearer (E-RAB).
Some data traffic may require a certain Quality of Service (QoS). In that case, a so called QoS profile may be associated with the radio access bearer. The QoS profile may comprise information relating to Guaranteed Bit Rate (GBR), Allocation and Retention Policy (ARP) and the like.
As an example, an eNB of the known LTE system may perform the admission control. In order for the eNB to determine whether or not to admit or reject a request for resources, the eNB is required to monitor resources free for use by data traffic and resources already used by data traffic. Hence, admission control considers resources which are herein referred to as Monitored System Resources (MSR). The MSR may relate to resource blocks, modulation and coding schemes, and the like.
There are two main categories of MSR; dynamic MSRs and static MSRs. Each MSR shall be considered as representing the aggregated resource usage from all traffic.
On the one hand, dynamic MSRs are resources for which the utilization may vary during a lifetime of a radio access bearer. The variations may stem from decisions by the eNB, i.e. a scheduler therein, as well as varying radio conditions or mobility of any user equipments served by the eNB. The utilization of dynamic resources is considered by admission control for E-RABs with a QoS requirement, e.g. Guaranteed Bit Rate (GBR) or a delay requirement.
On the other hand, static resources are resources for which the utilization does not vary during the lifetime of the radio access bearer. These resources may relate to licenses or hardware/software limitations, or be related to a static model of an otherwise dynamic MSR. A specific E-RAB request is mapped by Admission control on to the various MSRs; for a specific E-RAB request only some of the MSRs may be relevant.
Each MSR is characterized by a maximum capacity, such as a maximum number of bits that can be sent. If load, from E-RABs that already have been admitted, equals the maximum capacity on a particular MSR, further requests on the particular MSR are rejected. That is to say, the particular MSR is over-loaded. In other words, Admission Control rejects a request for an E-RAB requiring resources from any over-loaded MSR. Typically, dynamic MSRs are considered for load due to E-RABs with a challenged QoS requirement.
Some special considerations are needed for dynamic MSRs representing load due to E-RABs with a QoS requirement. If too many E-RABs with QoS requirements are admitted, scheduling by the eNB will at some point fail to provide resources to all of them. User equipments may have been admitted at a point in time when radio conditions and mobility were favourable in the sense that the desired QoS, or requested QoS, could be provided. But due to increasing mobility and worsened radio conditions the resources may at a later point in time not be enough to provide the desired QoS for the admitted E-RABs. Admission Control strives for having the load due to E-RABs with QoS requirements below a QoS threshold, which may be expressed as a percentage of the maximum capacity of the MSR.
It does so by rejecting E-RAB requests whenever load due to E-RABs with QoS requirements is above a so called QoS threshold. The QoS threshold may for instance relate to the contribution from all the Guaranteed Bit Rate Bearers (GBR Bearers).
A difference between the maximum capacity of the MSR and the QoS threshold is referred to as a margin. The margin allows for statistical fluctuations with regard to the air interface resources, since the load for high-priority QoS traffic can be limited to a value lower than the maximum level of the resource. Then integrity of the QoS E-RABs is protected with a level of probability since resources above the threshold are free for use by the high-prioritized traffic in congested scenarios. This means that the margin reduces the risk of dropped services on the E-RAB with QoS requirements. The risk will be one minus the level of probability. The level of probability is thus adjusted by setting the threshold to some desired value. Tuning the margin between the threshold and the maximum level of the resource is of special interest when the high-prioritized traffic consists of QoS traffic for which service blocking is desired rather than service dropping. As a consequence, it is desired to have a sufficiently large margin in order to ensure service blocking, i.e. a request for a radio access bearer is rejected, rather than service dropping, i.e. the eNB fails to schedule while fulfilling QoS requirements for an admitted radio access bearer.
In certain scenarios, when for example mobile TV traffic is broadcast by the eNB, some of the MSRs may be become occupied. The mobile traffic may utilize a Multimedia Broadcast/Multicast Service (MBMS) which was introduced in LTE Rel-9. Hence, this kind of traffic may be referred to as MBMS traffic. When some of the MSRs are occupied by mobile TV traffic, a problem may be that a radio access bearer, such as an E-RAB having a QoS profile and which has been admitted, may be dropped due to that the mobile TV traffic is prioritized over the traffic on the radio access bearer under consideration.